The Harvard-Smithsonian Center for Astrophysics reports on the discovery of a "reverse shock wave racing inward at Mach 1000" in the Tycho's supernova remnant, making its particles glow as you can see in the photograph above. If you think the idea of a shockwave travelling inward is counterintuitive, you aren't alone. This is what's happening.
When that ejecta rammed into surrounding interstellar gas, it created a shock wave - the equivalent of a cosmic "sonic boom." That shock wave continues to move outward today at about Mach 300. The interaction also created a violent "backwash" - a reverse shock wave that speeds inward at Mach 1000.
The reverse shock wave heats gases inside the supernova remnant and causes them to fluoresce. The process is similar to what lights household fluorescent bulbs, except that the supernova remnant glows in X-rays rather than visible light. The reverse shock wave is what allows us to see supernova remnants and study them, hundreds of years after the supernova occurred.
I imagine it like this
but according to Randall Smith — one of the paper's authors — "it's like the wave of brake lights that marches up a line of traffic after a fender-bender on a busy highway."